When I used C++ programs, I needed Turbo C complier; and when I have a Java program, I need to have JVM. Still C++ isn't platform independent, but Java is!
If any Java program require a JVM running in order to execute, why does Java is said to be Platform Independent?
No software is really "independent". Eventually, your program has to call the underlying OS in order to make some basic operations, like allocating memory, create new threads etc.
The way to achieve an executable which is "cross platform" is to create specific executable for each OS. Common practice is to write different code for each OS, and then "hide" it in a cross platform interface and compile the relevant code to the relevant OS. For example,
std::thread is "cross-platform" to the user who uses this class, but behind the scenes it will call different functions based on the OS which was specified on compile time (such as
CreateThread on Windows, but
pthread_create on *nix OS's).
So basically, the JVM is a C/C++ executable that was written with different set of functions for each OS, and was compiled separately for each OS. A JVM executable which works on Linux, will not work on Windows, and vice-versa.
That JVM compiles
.class files to machine code based on the OS and CPU it currently operating on, so that's why Java programs can "run anywhere".
But basically, it's a lie. It's like saying a human being can live on Mars.... if he lives inside a sealed spaceship with proper temperature, water, food, air and sunlight supply
So Java program can run anywhere.... if the JVM already is installed and running on the computer.